1. Technical Field
The invention concerns a device for testing a test object by means of a non-destructive measuring method. The test object is in particular a tire, however other components could also be tested using the device that is the subject-matter of the invention. The non-destructive measuring method with which the test object is tested is, for example, an interferometric measuring method. The device has at least one measuring head by means of which the test object is scanned to produce a measurement result. The device also has a positioning means by which the measuring head can be moved between a park position outside the test object and a measuring position located, for example, within the test object. In addition, the device is provided with a pressure chamber in which, during the test, the test object can be subjected to a prescribed pressure; the chamber has a hood. The device is also provided with a sub-frame on which the test object can be mounted during the test.
2. Description of Related Art
Tires or other component parts which are under load when in use are subjected to material tests to check their quality in order to reduce the safety risks; these tests enable faulty areas, known as defects, to be recognised. Above all, when used tires are to be remoulded, as a rule a non-destructive material test is employed which guarantees a comparatively fast series examination.
Optical measuring methods are frequently to be found in industrial practice such as for example holography or shearography, also commonly known as speckle pattern shearing interferometry. Shearography is a relative interferometric measuring method that produces a result image which shows the difference between two chronologically staggered conditions of the test object. In order to generate the resulting digital image used as a rule nowadays in view of the increasing application of electronic image sensors such as CCD- or CMOS-sensors, it is thus necessary to change the condition of the test object between two measurements by mechanical, thermal or pneumatic forces. To this end, known devices are provided with a pressure chamber which is either evacuated or pressurized so that the test object inside the chamber is deformed as a result of the pressure change and thus transfers from a first reference state to a second measurable state.
In contrast to holography, shearography does not determine the deformation on the surface of a test object but rather measures the gradients of deformation. This is attributable to the fact that shearography makes use of what is known as a shearing element which makes use of shear optics such as for example an optical wedge, an optical biprism or a Michelson interferometer, which generates image duplication. As a result of the shearing element, two slightly spatially-shifted images of the test object are produced and superimposed so as to generate an interferogram from the interference obtained. The shearogram characterised by the gradients of the deformation is created by subtraction of the intensities of the interferograms obtained in the reference state and the measured state. The shearogram indicates whether the position of a point in relation to a neighboring point has changed as a result of the deformation of the test object. If it has, then this positional difference leads to a local change in the intensity distribution which gives information about defects. Interferometric measuring methods which are based on this speckle-interferometry are described in DE 42 31 578 A1 and EP 1 014 036 B1.
The devices employed to test a test object by means of an interferometric measuring method generally have at least one measuring head which is provided with an illumination unit and an image-acquisition unit. The illuminating unit frequently consists of a coherent-light-emitting laser or laser diode. The image acquisition unit is usually a camera provided with an image sensor, i.e. a light-sensitive semiconductor sensor, for example a CCD- or CMOS-sensor. To achieve meaningful measurement results it is necessary to co-ordinate the camera's field of vision and the section of the test object that is to be tested. Generally, such co-ordination is achieved by positioning the measuring head in a measuring position and orienting it in an observation direction which ensure, on the one hand, that the section of the test object to be tested lies completely within the field of vision of the camera and, on the other hand, that sections to be tested and measured in sequence overlap each other sufficiently to enable a complete and thorough test. The measuring position and the observation direction of the measuring head depend on the dimensions of the test object. Accordingly, devices are known from EP 1 284 409 A1 and DE 103 33 802 A1 which enable the test object to be measured optically, for example by means of what are called light sections, so that the measuring head can be positioned and oriented depending on the data acquired in this way.
A tire testing device in which a tire to be tested without a wheel or rim is placed in a pressure chamber in a lying position is disclosed in DE 1 99 44 314 A1. The tire-testing device is provided with several measuring heads which can be positioned at a prescribed distance from the inner peripheral surface, the inner sidewall or the outer sidewall of the tire in order to test the substructure of the tire, i.e. the carcass, a belt frequently incorporated between the carcass and the running surface as well as the sidewall of the tire. The measuring heads each have an illuminating unit and an image acquisition unit and are positioned at an angle to each other so that different sections of the tire can be tested at the same time in order to perform the test comparatively quickly.
The measuring heads are connected to a positioning means which makes it possible to move the measuring heads from a park position outside the tire, which enables the tire to be tested to be changed, into a measuring position inside the tire to enable the inner peripheral surface and the inner sidewall of the tire to be tested. For this purpose the positioning means is provided with an arm that is movable around the longitudinal axis of the tire and on which the measuring heads are mounted. To enable the measuring heads to be brought into the necessary measuring position and into the desired observation direction, the measuring heads are movable in the radial direction of the tire and they can be swivelled around a pivot axle mounted on the arm.
The pressure chamber is formed with a hood which is arranged on a vertically movable slide. When the pressure chamber is in the closed position, the hood lies in a sealed position on the sub-frame on which the tire to be tested lies during the test. The hood is made in one piece and is, for example, of moulded material. Since the hood has to surround the tire to be tested, and in view of the comparatively large diameter of some tires to be tested, the dimensions of the hood would be such that it would have a significant weight. In this case, the hood would no longer be able to be handled without considerable trouble. The known testing device is thus, in practical terms, unsuitable for the testing of tires with a comparatively large diameter.
Tires with a comparatively large diameter are used mostly on heavy machinery. Heavy machinery such as industrial and agricultural vehicles or earth-moving machines are, as a rule, equipped with what are known as Off-The-Road (OTR) tires which have a comparatively large diameter of sometimes more than 5 meters. OTR tires have a high load-bearing capacity. For example, OTR tires can carry dumper trucks which are used for transport in mining activities or for freight transport at cargo ports and which can have loading capacities of more than 70 tons.
The use of OTR tires is associated with considerable risks. For example, the pressure wave generated by a burst of an OTR tire can cause considerable damage and can seriously endanger the safety of any person in the vicinity of the vehicle with the OTR tire in question. Moreover, the immobility of a vehicle fitted with OTR tires following tire damage is generally linked to very severe economic losses. It is frequently the case that mines and quarries, for example, have only single-track access roads and, consequently, if a vehicle used for carrying away the mined or quarried materials, mineral ore for example, breaks down, this can cause the whole mine or quarry to be temporarily brought to a standstill. The testing of OTR tires for flaws and defects is thus enormously important.